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How to build high sensitivity+high voltage function gen

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Dec21-13, 01:37 AM
P: 1,081
I just re-read the OP. I assume you have a function generator and a separate 100X amplifier that already gives you the high voltage you need. Are you saying you need different output impedances to get the measurements you need, not just different drive voltages.

1. If yes, what range of impedences do you need and how many steps?

2. If no, why not a fixed output resistance and a pot on the input to the amplifier.

I'm still not clear what your specs are with respect to drive voltage and drive impedance.
Dec21-13, 01:50 AM
Sci Advisor
P: 4,027
The OP was fairly garbled but it asked for a design of a suitable amplifier.

There was no description of the actual type of piezo being tested. It could be some kind of filter or an echo sounder transducer for a boat, or a tweeter for a sound system.

So, rather than tackle the difficult high voltage amplifier, it is easier to use a 10 cent resistor to measure the impedance.
Dec22-13, 09:36 AM
P: 46
The input impedance ranges from 50 to 50kohms. Drive voltage from 0.5 voltage to 100 V.

In piezoelectric materials, the impedance depends strongly on the driving voltage and the output vibration levels and the frequency. My aim is to test the impedance response using constant vibration conditions, which would require larger and larger voltages as you move farther away from resonance.

Please understand my question: I am asking how I can administer high voltages and low voltages in the same output. I sometimes need high voltages, so I cannot use an op amp, rather I am using a high speed power amplifier to amplify the signal. I need good sensitivity at the lower end, but I do not want to use two separate systems, so I use a pot between the func gen and the amp.

I believe the suggestion of using a relay between two function generators, one for high voltage and one for lower voltage, is the ideal solution. When I want to switch from high voltage to low voltage, I can bring down the voltage from the high power, then switch seamlessly to the low voltage func gen by matching the driving voltage.

Or I can use a MOSFET between the function generator and the amp. I can change gate voltage to change the input to the amplifier. What do you think about this? Which MOSFET could I use? Or I could use a digital pot
Dec22-13, 01:07 PM
Sci Advisor
P: 4,027
You seem to be describing a high voltage AC constant current sweep generator.

Assuming you don't have a spare $15000, I can't imagine anyone developing this for you for free.

So, maybe you could describe the results you have been getting so far with standard laboratory equipment. Are you doing this for your own interest or for an employer?

Could you do a manual sweep using a normal signal generator and then plot the results on graph paper?
You can maintain constant current by monitoring the voltage across a small series resistor and varying the voltage to keep this constant.
Dec22-13, 11:00 PM
P: 46
I use labview to automatically control the voltage to maintain constant current while changing the frequency in steps.

I am doing this as part of my Phd.

We have already built this system and got it to work and also published papers using it, but I am trying to improve it.

My need is this, I need to replace the pot in the diagram of the earlier post with something digital. The goal is to have fine control of the voltage at low voltage, but also have the capacity for high voltage output in the same system. I believe the solutions lie in what I explained in post #21. I am gonna try a MOSFET as a variable resistor tomorrow morning and see if I get reasonable results. I will let you know what happens.
Dec23-13, 06:17 PM
P: 46
It is clear that typical MOSFET's/low level transistors wont work because they are asymmetrical devices, and thus mangle AC sinusoidal signals.

I think we can call this solved. Relays or digital pots should both work. Thank you for all your help and ideas regarding my system and the voltage amplification.

Please reply if you have better ideas. If not, thank you so much for the helpful discussion.

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